In "A Separate Peace", how do the minor characters show that friendship reveals one's virtue (or lack of it)? What specific scenes demonstrate this?

Leper certainly wants to be friends with Gene. He calls Gene his best friend as he signed his telegram in the end of chapter 9. Leper uses the next chapter to totally unload on Gene about Leper's own problem of having gone AWOL. Leper's virtue reveals itself in the end of chapter 11. He feels Brinker's interrogating questions trying to lead him to blame Gene...

Let's take a look at two characters: Brinker and Leper.

Leper certainly wants to be friends with Gene. He calls Gene his best friend as he signed his telegram in the end of chapter 9. Leper uses the next chapter to totally unload on Gene about Leper's own problem of having gone AWOL. Leper's virtue reveals itself in the end of chapter 11. He feels Brinker's interrogating questions trying to lead him to blame Gene for jouncing Finny off the limb, but Leper won't implicate his friend Gene. His loyalty shines through. There are few virtues in life as important as that.

Brinker too wants to befriend Gene. Their relationship developed as Brinker had convinced Gene to enlist. They had such a plan to both enlist... until... Finny returned. Then Gene's hope to enlist died when Finny became depressed thinking that Gene would desert him. Gene couldn't let Finny down, which resulted in an attitude problem for Brinker.

In fact, Brinker, the most successful and well-rounded student went into a depression quitting all his school activities. Gene also reports that Brinker turned rebel "for the Duration." This revealed Brinker's lack of virtue.

A virtue is a trait to be valued. Although sometimes depression is just part of life, we don't long for it or value it.